1. Field of the Invention
The present invention relates to a method for forming a step on a deposition surface of a substrate for a superconducting device utilizing an oxide superconductor, and more specifically a to method for forming a step on a deposition surface of a substrate for a superconducting device such as a step edge type Josephson junction device of an oxide superconductor.
2. Description of Related Art
A Josephson junction device which is one of the most famous superconducting devices can be realized in various structures, for example, a stacked type (tunnel type) junction realized by a thin non-superconductor layer sandwiched between a pair of superconductors, a point contact type junction, a micro bridge type junction and a variable thickness bridge type junction which are composed of a pair of superconductor regions which are weakly linked to each other.
These Josephson junctions have fine structures so that fine processings are required to realize Josephson junction devices.
In order to realize a stacked type junction by using an oxide superconductor, a first oxide superconductor thin film, a non-superconductor thin film and a second oxide superconductor thin film are stacked on a substrate in the named order.
The thickness of the non-superconductor layer of the stacked type junction is determined by the coherence length of the superconductor. In general, the thickness of the non-superconductor layer of the stacked type junction must be within a few times of the coherence length of the superconductor. Since oxide superconductor materials have a very short coherence length, therefore, a thickness of a non-superconductor layer must be about a few nanometers.
However, the superconductor layers and the non-superconductor layer of the stacked type junction must be of high crystallinity for favorable junction properties, which are composed of single crystals or composed of polycrystals which are orientated in an almost same direction. It is difficult to stack an extremely thin and high crystalline non-superconductor layer on an oxide superconductor layer. Additionally, it is very difficult to stack a high crystalline oxide superconductor layer on the non-superconductor layer stacked on an oxide superconductor layer. Though the stacked structure including a first oxide superconductor layer, a non-superconductor layer and a second oxide superconductor layer is realized, the interfaces between the oxide superconductor layers and the non-superconductor layer are not in good condition so that the stacked type junction does not function in good order.
In order to manufacture a point contact type junction, a micro bridge type junction and a variable thickness bridge type junction by using oxide superconductor, very fine processings which realize a weak link of a pair of the superconductors are necessary. It is very difficult to conduct a fine processing with good repeatability.
The point contact type junction has been formed of two oxide superconductor thin films which are in contact with each other in a extremely small area which constitutes the weak link of the Josephson junction.
The micro bridge type junction has been formed of a constant thickness oxide superconductor thin film which is formed on a substrate and which is patterned in a plan view, so that a superconductor thin film region having a greatly narrow width is formed between a pair of superconductor thin film regions having a sufficient width. In other words, the pair of superconductor thin film regions having a sufficient width are coupled to each other by the superconductor thin film region having the greatly narrow width. Namely, a weak link of the Josephson junction in the superconductor thin film is formed at the greatly narrow width region.
On the other hand, the variable thickness bridge type junction has been foraged of an oxide superconductor thin film of a sufficient thickness which is formed on a substrate and which is partially etched or thinned in a thickness direction, so that a thinned oxide superconductor thin film portion is formed between a pair of superconductor thin film portions having the sufficient thickness. In other words, the pair of superconductor thin film portions having the sufficient thickness are coupled to each other by the thinned oxide superconductor thin film portion. Accordingly, a weak link of the Josephson junction is formed at the reduced thickness portion of the oxide superconductor thin film.
As would be understood from the above, characteristics of the Josephson junction device are a close relation to the contact area of the superconductor thin film in the point contact type Josephson device, the width of the superconductor thin film region having the extremely narrow width in the micro bridge type Josephson device, and the thickness of the thinned oxide superconductor thin film portion in the variable thickness bridge type Josephson device, both of which form the weak link of the Josephson junction. Therefore, in order to obtain the desired characteristics with a good repeatability, a high precision on a sub-micron level of the processing such as the etching is required.
The micro bridge type Josephson device can be said to be more preferable than the variable thickness bridge type Josephson device, since the micro bridge type Josephson device has a relatively planer surface, which is preferred in a integrated circuit. However, in order to form the weak link in the micro bridge type Josephson device, it is required to pattern an oxide superconductor thin film having the thickness on the order of 0.5 .mu.m to 1.0 .mu.m into a width of not greater than 0.2 .mu.m. However, it is very difficult to conduct this fine patterning with good repeatability.
On the other hand, in the variable thickness bridge type Josephson device, the very fine pattering is not required in order to form the weak link. However, it is very difficult to uniformly control the remaining thickness of the thinned portion forming the weak link. In addition, the variable thickness bridge type Josephson device cannot have a planer surface by nature. This is not preferable to the integrated circuit application.
In order to resolve the above mentioned problems, researches have been conducted to manufacture a Josephson junction device taking account of the characteristic advantages intrinsic to the oxide superconductor, which permits it to avoid the fine processing of the oxide superconductor.
The superconducting characteristics of the oxide superconductor considerably varys, depending on the crystalline direction. Particularly, the oxide superconductor has a large critical current density in the direction perpendicular to the c-axes of its crystals. Thus, if oxide superconductors having crystalline directions different from each other are joined together, a grain boundary at the interface becomes a barrier of the weak link so that a Josephson junction is formed. A Josephson junction device utilizing this Josephson junction is called an artificial grain boundary type Josephson junction device. A Josephson junction device of this type can be manufactured without the fine processing as mentioned above.
There is shown a sectional view of an example of the artificial grain boundary type Josephson junction device. The artificial grain boundary type Josephson junction device shown in FIG. 1 mainly consists of an oxide superconductor thin film 1 deposited on a deposition surface of an insulator substrate 2 having a step 23. A portion 13 of the oxide superconductor thin film I on the step 23 has a crystal orientation different from those of portions 11 and 12 of the oxide superconductor thin film 1 on deposition surfaces 21 and 22. By this, grain boundaries 51 and 52 are formed between the portions 11 and 13 and between the portions 12 and 13, which constitute a weak link of a Josephson junction.
The preferable height of the step 23 is considered to range from 100 to 500 nanometers and is determined by a thickness of the oxide superconductor thin film.
In order to manufacture the above Josephson junction device, in a prior art, the step of the substrate is generally formed by an ion milling using Ar ions. However, the deposition surface of the substrate is often degraded in its crystallinity by the ion milling process so that an oxide superconductor thin film having high quality can not be deposited on it. In addition, the ion milling is conducted under high vacuum, which requires a lot of time for evacuating, so that the deposition surface of the substrate can not be efficiently processed.
Furthermore, a niobium mask is sometimes used in the above processing of the deposition surface of the substrate, in order to form a steep step. Photoresist which is generally used for a mask of an etching process is rather soft so fix at an edge of a photoresist film gently rises. In case that the deposition surface of the substrate is etched by the ion milling with using a photoresist mask, a step gently rising is formed on the deposition surface. If an oxide superconductor thin film is deposited on the deposition surface having the step gently rising, no grain boundary might be formed in the oxide superconductor thin film. Since niobium is far harder than the photoresist, an edge of a niobium film abruptly rises. By using a niobium film as a mask, a steep step can formed on a deposition surface of a substrate. However, niobium may be remain on the deposition surface of the substrate even after the niobium film is removed. The remaining niobium has an adverse effect on an oxide superconductor thin film deposited on the deposition surface of the substrate.